In recent years, flat panel displays such as liquid crystal displays or plasma displays are broadly used in view of saving space and realizing high definition. Among these, the liquid crystal displays have attracted attention since they can save electricity and realize high definition, and developments thereof are ongoing.
A polarizer serving as an optical shutter is used along with liquid crystals in a liquid crystal display panel. The polarizing plate is provided with a polarizer and is an essential component of the liquid crystal display panel. In general, polarizers are prepared by uniaxial stretching a polyvinyl alcohol resin in a water bath by 5˜6 times, thus conventional polarizers tend to be easily torn in stretched direction and have low hardness. For this reason, polarizing plates prepared by attaching a protective film to the surface and/or back of the polarizer, are used in the art. When an adhesive used to attach a protective film to a polarizer for be applied to a polarizing plate, the adhesive is required to meet specific conditions.
Japanese Patent Laid-Open Publication No. 2005-036077 discloses an adhesive for polarizing plates prepared by curing a copolymer prepared by copolymerizing multivalent carboxylic acid or an anhydride thereof and a monomer having epoxy groups through electron beam irradiation. In addition, WO 2005/085383 discloses an aqueous adhesive for polarizer devices containing a polyvinyl alcohol resin, a resin containing a maleic anhydride skeleton in its structure, and a crosslinking agent.
Further, Japanese Patent No. 456139 discloses a photocurable adhesive including: a base resin consisting of a radical polymerizable compound, which mainly includes a monofunctional radical polymerizable compound, and a cationic polymerizable compound, which mainly includes a cationic polymerizable compound containing no (meth)acryloyl group: a photoradical polymerization initiator; and a photocationic polymerization initiator.
Polarizers generally contain moisture since the polarizers are prepared through stretching in a water bath as set forth above. The cationic polymerizable compound used in the art is cured at some period of time after ultraviolet irradiation and the like. In this case, curing may be inhibited by moisture, thereby generally deteriorating initial curing properties. This problem becomes more conspicuous when polarizing plates are prepared particularly under high humidity. On the other hand, since curing of the adhesive using the radical polymerizable compound is not inhibited, initial curing properties are good. However, the adhesive has a disadvantage in that adhesion between the polarizer and the protective film is slightly deteriorated.
Furthermore, the adhesive of Japanese Patent No. 456139 employing the cationic polymerizable compound and radical polymerizable compound fails to overcome the problem of curing inhibition due to moisture, and thus has insufficient initial curing properties and adhesion.